单项选择题

When ancient farmers first domesticated wheat some 10,000 years ago, they were trying to breed a crop perfectly suited to making food. But it seems that somewhere along the line, farmers unwittingly(无意地) took out a gene that packs the plant full of goodness. Now researchers have found a way to fix that old mistake and put the nutrients back.
A team led by Jorge Dubcovsky of the University of California, looked at more than 90 varieties of the two major types of wheat that are today cultivated for making pasta and bread--foodstuffs that account for around 20% of all calories consumed worldwide. In all of these strains, the researchers found a mutation(突变) that reduces the amount of protein, zinc and iron in the grain.
The researchers have successfully crossed wild and crop strains to create a new variety with increased micronutrient (微量营养素) levels and 10% more protein than traditional farm strains. The discovery could help many of the estimated 2 billion people with deficiencies in key micronutrients. "We’re releasing a variety that will be on the market next year," says Dubcovsky. "We’ve tested the quality and it makes very good bread."
Because the gene, called NAM-B1, comes from a wild relative of wheat, there is no need to use transgenic methods to transfer the gene into crop strains, Dubcovsky adds. "You don’t need to do any biotech--you just cross and restore it," he says.
The gene speeds up the process of "senescence"(衰老), in which the plant’s leaves die off and nutrients are remobilized to the developing grains. The mutated version in traditional cultivated strains results in fewer nutrients making it into the grains by the time the crop is harvested, the researchers explain.
They tracked down the gene responsible by using a map of genetic markers showing which areas of the genome are associated with different traits. Having identified the region associated with higher nutrient levels, they analyzed the genetic sequence to find the crucial genes.
To test their theory, Dubcovsky’s team used a gene-silencing technique called RNA interference to shut down all genes related to NAM-B1 in crop strains, and found that the nutritional content of the grains declined even further.
The new strain is "not a magic bullet", Dubcovsky stresses. "You will still have to test it in your own environment." He and his colleagues are set to release their variety onto the U.S. market, but if other regions were to follow suit, wild strains would need to be crossed with local crop varieties to ensure that the result is suited to growing in local conditions. Other factors, including soil quality, can also influence the health value of plants.
The word "mutated" (Line 2, Par